1. Field of the Invention
The present invention relates to an ignition device for an internal combustion engine and, particularly, to an ignition device for an internal combustion engine capable of preventing the reduction of discharge energy of a spark plug and of suppressing noise caused by LC resonance after the completion of discharge of the ignition coil.
2. Prior Art
In an internal combustion engine using gasoline as a fuel, the gas mixture compressed by a piston is ignited by an electric discharge of a spark plug. In an ignition device that is generally used, a high voltage of 20 to 30 KV induced in the secondary coil when a current flowing into the primary coil of the ignition coil is interrupted is supplied to the spark plug.
In fact, however, energy accumulated in the primary coil or in the secondary coil is not completely consumed by the electric discharge of the spark plug, and surplus energy causes LC resonance, after the ignition, due to parasitic inductance and parasitic capacitance of a high-tension cable that connects a distributor to the spark plug. The LC resonance affects various devices as noise.
FIG. 1 is a diagram schematically illustrating an ignition circuit for an internal combustion engine, wherein an end of a primary coil 111 of an ignition coil 11 is connected to the positive electrode of a battery 12, and the other end is grounded through collector and emitter of a switching transistor 13 included in an igniter.
The base of the transistor 13 is connected to an ignition timing control unit 14. The transistor 13 is turned on when an ignition signal IGT is output from the ignition timing control unit 14.
One end of a secondary coil 112 of the ignition coil 11 is also connected to the positive electrode of the battery 12, but its other end is connected to the spark plug 18 through a reverse current-preventing diode 15, a distributor 16 and a high-tension cable 17.
When the ignition signal IGT from the ignition timing control unit 14 is turned on, a pulse generated in the secondary coil 112 is blocked by the reverse current-preventing diode 15. When the ignition signal IGT is tuned off, however, a pulse generated in the secondary coil 112 passes through the reverse current-preventing diode 15 so that an electric discharge takes place on the spark plug 18.
A device that will be affected by the noise due to LC resonance may be an ionic current detector for detecting a current that flows through ions generated by the combustion of a mixture gas.
The ionic current detector 19 is connected in parallel with the spark plug 18 at the output side of the distributor 16.
An ionic current is guided, through a protection diode 191, to a series circuit consisting of a current-voltage converting resistance 192 and a bias power source 193. A voltage generated at a point where the current-voltage converting resistance 192 and the protection diode 191 are connected together is guided, through a DC component-cutting capacitor 194, to an amplifying circuit 195 composed of an operational amplifier and resistors.
Therefore, a voltage signal proportional to the AC component of the ionic current is outputted at the output terminal 196 of the ionic current detector 19.
However, the ionic current is so weak that the amplifying circuit must have a large gain and a large input impedance, and the amplifying circuit may be easily affected by the external noise. In order to solve this problem, "an ionic current detector" in which the output of an ion detector 19 is masked while LC resonance is taking place has been already proposed (see Japanese Unexamined Patent Publication No. 6-299941).
When the engine speed increases, however, LC resonance period becomes so close to the period for observing ionic current for detecting knocking or misfiring that it becomes difficult to control the timing for opening and closing the mask.
In other words, though the LC resonance period is not affected by the engine speed, the period for observing the ionic current approaches the LC resonance period in accordance with an increase in the engine speed.
Besides, if the LC resonance is masked, the LC resonance is not substantially removed, and the devices other than the ionic current detector are not free from being affected by noise caused by LC resonance.
It is advantageous to control the LC resonance itself after the electric discharge of the secondary coil of the ignition coil. For this purpose, it can be contrived to provide a so-called snubber for absorbing LC resonance in parallel with the primary coil or the secondary coil of the ignition coil.
FIG. 2 is a diagram to explain ways to provide the snubber. There can be contrived two ways, the first way provides the first snubber 21 constituted by a diode and a resistor connected in series (or a resistor and a capacitor connected in series) in the primary coil 111, and the second way provides the second snubber 22 which is a resistor in the secondary coil 112.
The first snubber 21 must have a diode for improving the efficiency for transferring energy accumulated in the primary coil 111 to the secondary coil 112. When the voltage across the diode becomes lower than the forward voltage drop of the diode (about 0.6 V), however, the effect of snubber for absorbing the LC resonance is no longer exhibited.
The second snubber 22 consumes part of the energy, inevitably causing a decrease of the ignition energy. Besides, the voltage across the second snubber reaches 20 to 30 KV and, hence, the device itself must have a high breakdown voltage.
The present invention is accomplished in view of the above-mentioned problems, and provides an ignition device for an internal combustion engine capable of suppressing noise due to LC resonance after the discharge of the ignition coil without decreasing discharge energy of the spark plug.